The present invention relates to a focus detection and processing method and apparatus employing a double-image coincidence technique by which an object's image is divided into two parts, the two parts are displaced in opposite directions or one of the two parts is displaced. When the object is in focus, the relative displacement of the two parts becomes a minimum. The outputs of a pair of photoelectric conversion element groups (hereinafter referred to as "sensor groups", when applicable) are processed to electrically detect the focusing position. More particularly, the invention relates to a focus detection and processing method and apparatus for a camera in which the sign of a focus detection output is changed before and after focusing. Not only front and rear foci are detected, but also an amount of movement of the photographing lens to the focusing position is instantly detected wherever the photographing lens is positioned.
A number of focus detecting techniques utilizing variations of spatial frequency, variations in contrast of an object's image or the above-described double-image coincidence technique have been proposed in the art and some of them have been put into practical use.
However, most of the conventional focus detecting techniques are disadvantageous in the following points. In these systems, when an object is in focus, the focus detection output has an extreme value, either a maximum value or a minimum value, and hence front and rear focus information is not available. Accordingly, in order to detect a focusing position, the focus detecting operation must be carried out over an entire photographing range of from infinity to the closest focusing distance. Thus, the operation of the device is intricate and the required detection time is long with the result that focus detection is impossible for a quickly moving object.
A focus detecting technique capable of providing front and rear focus information has been also proposed in the art. However, even with this technique, it is difficult to sufficiently carry out focus detection for a quickly moving object. Accordingly, the use of the camera is greatly limited and it is difficult to make the optimum use of all the functions of the camera.